1. Field of the Invention
The present invention generally relates to flexible electrical connectors, and more specifically to a flexible cable carrying electrically conductive traces with dots formed thereon to facilitate connection of the traces to a mating connector.
2. Description of the Related Art
Flexible cables carry electrically conductive traces for interconnection of electrical circuit boards and components. Electrically conductive dots or buttons may be formed on end portions of the traces for mating with conjugate traces on another connector formed on, for example, a circuit board. An example of this type of arrangement is found in U.S. Pat. No. 4,453,795, entitled "CABLE-TO-CABLE/COMPONENT ELECTRICAL PRESSURE WAFER CONNECTOR ASSEMBLY", issued June 12, 1984, to Norbert Moulin.
Pressure is applied to mate the cable to the conjugate connector such that the buttons on the cable are pressed against the traces on the mating connector to effect electrical contact therebetween. However, the cable is conventionally formed of at least one flexible material which is subject to plastic deformation, such as an adhesive which exhibits creep in response to applied pressure. Even if a resilient pad is used to press the connectors together with a relatively even pressure distribution, the material of the cable under the dots deforms plastically, and the dots progressively recede into the cable.
As shown in FIG. 1 of the drawing, a prior art electrical connector assembly 10 is generally designated as 10. A flexible cable 12 of the assembly 10 includes a base layer 14 formed of a flexible material such as polyimide, and electrically conductive traces 16 adhered to the base layer 14 by an adhesive layer 18 made of, for example, an acrylic adhesive. A coverlay 20 including a polyimide layer 22 and adhesive layer 24 is formed over the surface of the cable 12 and traces 16.
The cable 12 further includes electrically conductive dots or buttons 26 formed on selected traces 16, which are ohmically connected to the respective traces 16 and extend through the coverlay 20 beyond or external of the surface thereof. In other words, the dots 26 protrude beyond the surface of the coverlay 20 by a desired height.
Further illustrated is a connector member such as the edge of a circuit board 28, with which the cable 12 is designed to mate, which carries electrically conductive traces 30 conjugate to the dots 26. The facing surfaces of the cable 12 and circuit board 28 are pressed together such that the dots 26 ohmically contact the traces 30. Pressure is applied by a pressure plate 32 including a rigid back 34 and a resilient pad 36. The dots 26 may be somewhat deformed by the applied pressure to enhance the ohmic connection with the traces 30, whereas the pad 36 serves to make the applied pressure as uniform as possible over the area of the cable 12.
The problem with the prior arrangement of FIG. 1 is illustrated in FIG. 2. The pressure applied to the cable 12 in the area under the dots 26 is necessarily greater than over the main area of the cable 12, due to the protrusion of the dots 26 beyond the surface of the coverlay 20. This results in localized plastic deformation of the adhesive layer 18 and possibly the polyimide layer 14 under the dots 26, and recession of the dots 26 into the cable 12. The height of the dots 26 above the surface of the cable 12 decreases, resulting in reduced pressure between the dots 26 and mating traces 30 on the circuit board 28, and deteriorating electrical contact therebetween.
Plating the dots to a greater height in an attempt to relieve this problem is difficult in practical application. The inner walls of the holes and the exposed surfaces of the traces at the bottom of the holes must be chemically cleaned prior to plating to remove organic materials and oxides. Increasing the plating height of the dots increases the aspect ratio (height/diameter) of the holes, making it increasingly difficult for the cleaning solutions to completely enter therein. In addition, the higher aspect ratio makes it difficult to plate the dots into the holes, which constitute blind vias, due to the increased risk that entrapped bubbles will form in the holes and prevent complete formation of the dots. An entire flexible connector must be discarded if even one dot is not formed properly, since reworking of such a connector is not possible.